BUG/MEDIUM: conn-stream: Don't erase endpoint flags on reset

BUG/MEDIUM: conn-stream: Don't erase endpoint flags on reset

Only CS_EP_ERROR flag is now removed from the endpoint when a reset is
performed. When a new the endpoint is allocated, flags are preserved. It is
the caller responsibility to remove other flags, depending on its need.

Concretly, during a connection retry or a L7 retry, we must preserve
flags. In tcpcheck and the CLI, we reset flags.

This patch is 2.6-specific. No backport needed.